Understand your size measurements 

 

e = maximum overall section width
D = external tyre diameter (2 x R)
Ø = nominal bead seat diameter
H = section height
S = section width on measuring rim
   (this rim is indicated in bold faced type)
E = minimum dual spacing  (on measuring rim)
R = free radius
R’ = static loaded radius

Understanding your tyre sizes

 

There are several ways to specify a tyre size, with measurements sometimes given in millimeters, sometimes in inches, or sometimes a combination of both! The tyre size contains always the tyre’s section width and the rim size. The tyre’s overall diameter is not always shown in the dimensions, but can be calculated from the other dimensions plus their aspect ratio. Four common examples and explanations of tire measurements & sizes given below will help you to understanding these values will help you search for your tyres.

EXAMPLE: 23.5R25

  • ‘23.5’ is the section width of the tyre in inches.
  • The ‘R’ symbolizes a radial construction, whereas a ‘-‘ instead would mean its crossply or solid
  • ‘25’ is the wheel/rim diameter in inches.

There is no aspect ratio (H/S) indication included in the tyre size, but with this type of marking the tyre sidewall height is normally taken to be 80% of the tyre width. So its assumed the H/S ratio is approximately 0.80.

So, a 23.5R25 is a radial tyre:

– 23.5 inches wide
– sidewall height of 18.8 inches (80% of 23.5 inches)
– overall diameter of 1625 (2 x Sidewall Height + Rim)
– fits a 25″ rim/wheel

EXAMPLE: 875/65R29

  • ‘875’ is the section width of the tyre in mm,
  • ’65’, is the profile of the tyre (height of the sidewall expressed as a percentage of the width),
  • the ‘R’ symbolizes a radial construction
  • ‘29’ is the rim size in inches

 


So a 875/65R29 is a radial tyre:

– is 875 mm wide
– has a profile of 65% (sidewalls are 65% of 875mm = 1137 mm)
– overall diameter of 1875 mm (2 x Sidewall Height + Rim)
– fits a 29 inch rim

Example: 27.00R49

  • ‘27.00’ is the section width of the tyre in inches. (+ profile number percentage indication)
  • The ‘R’ symbolizes a radial construction
  • ‘49’ is the wheel/rim diameter in inches.

So a 27.00R49 is a radial tyre:

  1. 27 inches wide
  2. H/S ratio is 1 (100 series)
  3. sidewall height of 27 inches (100% of 27 inches)
  4. overall diameter of 2690 mm (2 x Sidewall Height + Rim)
  5. fits a 49″ rim/wheel.

Example: 33×12-20

  • ‘33’ is the outside diameter of the tyre in inches.
  • ‘12’ is the section width of the tyre in inches,
  • ‘20’ is the rim size in inches.
  • The ‘-‘ symbolizes a Crossply of Solid construction.

So a 33×12-20 is a solid tyre:

  1. 33 inches tall
  2. 12 inches wide
  3. Solid constructed
  4. fits a 20″ rim/wheel.

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